Exercise machines are known in which various devices are used to simulate a weight stack to provide a counterforce to a user of the machine. One known exercise machine employs a hydraulic cylinder to apply a counterforce to an exercise bar engaged by the user wherein a computer controls the force applied by the hydraulic cylinder. Another known type of exercise machine employs an alternator or generator coupled to an electrical load resistor to apply a counterforce to the user. In this type of machine, as the rotational velocity of the alternator or generator increases, so does the counterforce felt by the user. These known types of exercise machines are very costly and do not adequately simulate a weight stack.
To provide an economical exercise machine, it is desirable to use a DC motor to provide a counterforce which simulates a weight stack. Attempts have been made to control a DC motor to apply maximum force against the user as soon as the user interface or exercise bar is displaced. Such attempts have included applying a maximum control voltage to the DC motor immediately upon sensing displacement of the user interface. However, because the control voltage goes from zero to a maximum value almost instantaneously, such controls are very unstable. Further, such exercise machines typically dissipate the same amount of power when the user interface is at rest as when the interface is at its maximum displacement.